--- rcirc.el	2012-07-25 23:52:41.813226461 +0800
+++ rcirc-1.el	2012-07-25 23:55:20.813220626 +0800
@@ -792,21 +792,40 @@
 (defvar rcirc-max-message-length 420
   "Messages longer than this value will be split.")
 
+(defun rcirc-multibyte-position-at-byte (str bytes)
+  (if (multibyte-string-p str)
+      (rcirc-multibyte-position-at-byte-1 str bytes 0 0)
+      bytes))
+
+(defun rcirc-multibyte-position-at-byte-1 (str bytes now-chars now-bytes)
+  (let ((len (length str)))
+    (if (<= len 1)
+        now-chars
+      (let* ((half-len (/ len 2))
+             (lstr (substring str 0 half-len))
+             (rstr (substring str half-len len))
+             (now-bytes-1 (+ now-bytes (string-bytes lstr))))
+        (if (> now-bytes-1 bytes)
+            (rcirc-multibyte-position-at-byte-1 lstr bytes now-chars now-bytes)
+          (rcirc-multibyte-position-at-byte-1 rstr bytes (+ half-len now-chars) now-bytes-1))))))
+
 (defun rcirc-send-message (process target message &optional noticep silent)
   "Send TARGET associated with PROCESS a privmsg with text MESSAGE.
 If NOTICEP is non-nil, send a notice instead of privmsg.
 If SILENT is non-nil, do not print the message in any irc buffer."
   ;; max message length is 512 including CRLF
   (let* ((response (if noticep "NOTICE" "PRIVMSG"))
-         (oversize (> (length message) rcirc-max-message-length))
+         (oversize (> (string-bytes message) rcirc-max-message-length))
+         (adjusted-pos (if oversize
+                            (rcirc-multibyte-position-at-byte message rcirc-max-message-length)))
          (text (if oversize
-                   (substring message 0 rcirc-max-message-length)
+                   (substring message 0 adjusted-pos)
                  message))
          (text (if (string= text "")
                    " "
                  text))
          (more (if oversize
-                   (substring message rcirc-max-message-length))))
+                   (substring message adjusted-pos))))
     (rcirc-get-buffer-create process target)
     (rcirc-send-string process (concat response " " target " :" text))
     (unless silent